The enhancer element of the human histo-blood group ABO glycosyltransferase gene has been demonstrated to be located -3.7 kb upstream from the transcription start site and to be composed of four tandem repeats of a 43-bp unit. Recently we identified three different enhancer structures among the allelic A, B, and O glycosyltransferase genes. The enhancer structure with four 43-bp units is present in the B and O genes, but not in the A gene. The corresponding enhancer region of the A gene contains only one 43-bp unit, and within this unit a nucleotide substitution exists when compared with the consensus sequence. Through transient transfection assays, the transcriptional activity of the A-gene enhancer region was demonstrated to be less than 1% of that of the B-gene enhancer. The difference between the transcriptional activities of the two enhancers became more significant when acting in concert with the ABO-gene's native promoter. The different repeat numbers of the 43-bp unit possessed by the two allelic genes were shown to be the main reason for the vast difference in the transcriptional activities between the A-gene and B-gene enhancers.